Touring the Skies By: Jim Bonser (jbonser@usa.net)

August 1, 2014

Ps. 19:1 The heavens declare the glory of God; the skies proclaim the work of his hands. Okay all of you budding astro-photographers, this month we are going to be treated to a special full moon commonly called a Super Moon on August 10th. So get your cameras ready and start looking now for an interesting place to take a picture of the rising moon. What makes this month’s full moon “super”? Well, to answer that question we have to know just a little bit about orbital mechanics. Don’t worry, I will not get into the math or too far into the weeds, but I would like to take us on a short “history of astronomy” journey that will help us understand and appreciate this month’s ‘super moon’ just a little better. I’d like to start our little trip down Astronomy-memory Lane with an ancient astronomer named Aristarchus. He lived between 310 and 230 BC. Aristarchus was trying to come up with an explanation for the motion of the outer planets (Mars, Jupiter and Saturn). Why did they seem to move westward and then eastward compared to all the other stars in the sky? At the time, Greek philosophers believed that everything in the heavens was perfect and immutable. And since the most perfect shape was a perfect circle, then naturally the planets must orbit in perfect circles. Most people at the time also believed that Earth was the center of the solar system and everything including the Sun orbited Earth. Aristarchus thought that if the Sun, which he had calculated to be many times the size of Earth, were placed in the center of the solar system then the motions of the other planets would make sense. Our perspective as we orbited the sun looking at the planets would make them appear to move westward in the sky as we overtook them in our orbit. As we passed and began to move farther and farther away from them, they would appear to slow down and then resume their easterly motion compared to the stars in the distant background. Aristarchus had it right, of course, but he could not imagine that the planets orbited in anything but perfect circles. Unfortunately, because he assumed that the orbits must be perfect circles, the calculations did not match the observations and his model did not accurately predict the actual positions and motions of the planets. After Aristarchus came Claudius Ptolemy who lived from about 90 AD to 168 AD. Since Aristarchus’ system did make some sense but was not quite accurate enough, the Geeks decided to put Earth back in the center and make the planets travel in little circles in their orbits around Earth. Ptolemy did not invent this system, but he perfected it to the point that it was able to predict the motions and positions of the planets with very good accuracy and it remained unchallenged for over a thousand years. Eventually, though, better instruments were developed that made more accurate measurements of the planet’s positions and the Ptolemaic model became so complex that people started looking for a simpler explanation. Copernicus published a work that gave convincing proof for the Sun to be at the center of the solar system, but it had problems and even he had to resort to using those little circle (epi-cycles) to make it work. Various others tried to refine the motions including Tyco Brahe and Johannes Kepler. Kepler came up with the idea that if the orbits of the planets were ellipses (egg shaped) instead of circles, then things worked much better. Finally Sir Issac Newton brought it all together and we finally had a mathematical system that could simply and accurately describe the orbits and motions of the planets with the Sun in the center of the solar system and the planets in the order that we know to be true today. This brings us back to the Super Moon. As Kepler discovered, the Moon orbits Earth not in a circle, but in an ellipse. This means that at one point in its orbit the Moon will be closer to Earth than any other time. We call this point perigee. When things are closer to us they appear larger and that is the case with the Moon and with planets as well. In the case of the Moon, if it happens that the moon is a full moon at the same time that it is at perigee then that is what is known as a “Super Moon”. How much larger will it appear? Well, the smallest full moon of 2014 happened on January 15. It was 252,607 miles from Earth. On August 10 when the Moon reaches perigee at 3 PM it will be 221,765 miles from the center of Earth. You have to remember that astronomers calculate distances from the centers of things so from the surface of the Moon to the surface of Earth will actually be about 5,000 miles closer than these numbers indicate. Anyway, this means that the August 10 Moon will be about 13% bigger than the January 15 Full Moon and that also means that the moon will be about 26% brighter! So, pay attention to where the moon rises a few days before the 10th. And then scout out an interesting object for the Moon to rise over or near such as an interesting building or bridge or something and then setup your camera and take some amazing pictures! Remember that to make the moon look bigger relative to things like trees and buildings, you will need to be far away and use a telephoto lens to ‘bring them together’. Clear Skies!